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Well, from your symptoms it seems you are suffering from Panic Attacks , which is characterised by features like fearfulness, feeling of impending doom , breathlessness, chest tightness, increased and pounding heart beat felt by the person herself(palpitation), etc , and if you compare these characterstic features with your own set of symptoms , you will find that you quite fit the diagnosis of Panic Attack.
I can understand , you must be feeling the dread of symptoms every now and then , as these panic attacks symptoms simulate a heart attack or other serious pathological condition, but do not worry , Panic Attacks are very much treatable .
I am mentioning below treatment regime which you need to discuss with your doctor and get startd on , i am quite sure you will benefit from it , and your panic symptoms will be controlled.
1) 1 Anti anxiety drug needs to be started , as panic attacks are a type of Anxiety Disorder. Some good examples of Potent Anti anxiety drugs are -- Klonopin ( clonazepam) , Buspar (Buspirone ) , Etizolam . Discuss their doses and time of intake , and ask your dostor to choose any one of them , and get started on it. And to allay any kind of apprehension of getting side effects of these drugs , then rest assured , these are safe drugs , but i would suggest you ask your doctor to start you on a mild dose only , and after 1 month of taking it , and seeing the response , he may increase the dose, if at all required .
2) 1 Anti depressant , to be added to the above mentioned anti anxiety drug , this combination will take care of your symptoms . Last time you had tried Zoloft ( sertraline ) which is an SSRI anti depressant , and you did not find it to be good , so This time i suggest you to start on a different better anti depressant called asEffexor XR ( VENLAFAXINE extended release ) capsule , and this is to be taken only once a day , in the morning , as it is extended release , so do mention to your doctor Venlafaxine XR , AS VENLAFAXINE plain is also available , but the venlafaxine XR has got longer duration of action, and works throughout the day. Discuss the dose of this drug with your doctor, and again it has less side effects than other anti depressants.
So, i suggest you discuss with your doctor the above 2 mentioned drugs and get them prescribed and get placed on them , you need to be patient for the initial 1-2 weeks with these drugs , as this much time these drugs will take to reach their maximum action in the body , and from there on the positive results of these drugs will be there to notice.
As for breathing difficulty during a panic attack , i suggest you try breath in and breath out from a paper bag , putting it close to your mouth and breathing in and out .This will allay your breathlessness.
I am mentioning below a few links for you to get more information about panic attack (PA).
I wish you best of health .
I hope my answer serves your query according to your satisfaction.